Destructive Interference Mediated Topological Transitions in Bilayer Metasurfaces

Optical bound states in the continuum (BICs) are polarization singularities with integer topological charges (ν) in momentum space, whose far-field radiation vanishes in the surrounding radiative states. Here, we study the dynamic evolution of topological charges in symmetry-protected BICs within bilayer metasurfaces. Governed by threefold rotation symmetry (C_{3}), the topological charges obey ν=3n+1 (n∈Z). Theoretically and experimentally, we demonstrate that merely tuning the spacer thickness of the bilayer metasurfaces induces a topological transition from a fundamental charge (ν=1, n=0) to a high-level charge (ν=-2, n=-1) without modifying symmetry. Perturbation theory reveals this transition occurs when the first derivative of the far-field radiation with respect to the Bloch wave number k vanishes at Γ, induced by destructive bilayer interference. This interference simultaneously enhances the quality factors of near-Γ resonances and alters their scaling laws from O(k^{-2}) to O(k^{-4}). Our work introduces a symmetry-preserved mechanism to dynamically control BIC topology.